Some common types of high intensity discharge lamps used for general lighting comprise a quartz or fused silica arc tube enclosed within a glass outer jacket fitted with a screw base at one end. In high pressure mercury vapor lamps the arc tube contains a filling of mercury, whereas in high pressure metal halide lamps, the arc tube contains a filling of mercury and metal halides. In both kinds, the arc tube transmits ultraviolet radiation which is harmlessly absorbed by the glass outer envelope, or even gainfully absorbed by a phosphor coating on the outer envelope.
Most commonly the outer envelope remains intact to the end, and lamp life is ended by other factors. However it does happen occasionally that the outer envelope or glass jacket is shattered and the arc tube remains intact so that the lamp continues to operate. In this mode of operation, the ultraviolet radiation from the arc tube is not intercepted and may create a safety hazard.
A solution proposed to this problem by my application Ser. No. 601,858, filed Aug. 4, 1975, now U.S. Pat. No. 4,013,919, titled "Discharge Lamp Having Fuse-Switch Guard Against Jacket Failure" and assigned like this application, comprises a fuse located in the outer envelope which is connected in series with the arc tube and shunted by a thermal switch. In normal operation the switch is maintained closed by heat from the arc tube and prevents waste of energy in the fuse. Failure of the jacket while the lamp is on allows air to enter and cool the switch so that is opens. Lamp current is then drawn through the heater which rapidly oxidizes and opens the circuit, thereby disabling the lamp. While this arrangement is effective and achieves its intended purpose, a more economical solution is desired.